Geology Reference
In-Depth Information
(a)
(b)
Figure 4.59
Bottom of an algae‐infested freshly sampled 100 mm diameter FY sea ice core from Resolute
Passage in early May 1991 shown in (a); 5 mm thick vertical section of the core bottom is shown in (b) with
scale in millimeters (N. K. Sinha, unpublished).
Figure 4.59a is an illustration of the bottom section of
a 1.86 m long and 100 mm diameter freshly sampled,
algae‐infested core of FY ice with vertically oriented S3
type of columnar grain structure. The bottom of the core
clearly shows the green‐brown zone of algae infestation
up to about 40 mm above the core bottom or the ice‐
water interface. A 5 mm thick and 20 mm long vertical
section was cut from the middle of the core bottom for
examining the extent of the algae infestation. Figure 4.59b
indicates that the concentration of algae population
decreases rapidly with the increase in distance from the
ice‐water interface.
Growth in the population of algae is naturally expected
to be confined along the brine‐rich spaces between the
dendrites at the skeletal layer and partially protuding in
the water (see Figure 6.17 in Chapter 6). As the ice and
hence the dendrites grow slowly during the spring, the
algae must be incorporated along the boundaries of the
dendrites or platelets in zones adjacent to the skeletal
layer. In other words, algae population must be confined
within the boundaries of the subgrains. The presence of
this biomass must change the usual characteristic of the
grain and subgrain boundaries. To test this hypothesis,
thin sections were prepared by using the solid‐state DMT
described in section 6.2.2. The sections were prepared
inside a field laboratory at about −15 °C.
Figure 4.60a is a cross‐polarized light view of a double‐
microtomed horizontal thin section from a thick section,
within or close to the skeletal layer, cut at about 12 mm
above the bottom of an ice core. A vertical thin section
prepared from the central area of the bottom 12 mm of
the same core (immediately below the horizontal section)
is also shown in this photograph. The boxed area in
Figure 4.60a is shown in Figure 4.60b after a small angu-
lar rotation of the cross polarizers. No doubt, infestation
of algae are limited along the numerous subgrain bound-
aries of the grains.
Details of a number of grains in the horizontal sec-
tion shown in Figure 4.60a, with positions of the polar-
izers rotated to get the best views of the subgrains are
shown in Figure 4.61. Note the presence of the algae
cells, with appearances of round or elliptical shapes.
These cells are along the length of the parallel rows of
subgrain boundaries. Since the subgrains are oriented
parallel to the basal planes of the grains, the orienta-
tions of the algae cells also indicates the direction of the
basal plane and hence the direction of the
c
axis. The
microstructural information presented in this and simi-
lar figures confirms beyond any doubt that the presence
of the brine along the boundaries between the grains
and subgrains in FY ice, particularly in the skeletal
layer, serves as habitat for ice‐specific organisms, which
constitute the base of the food web for marine species.
These include not only algae but also phytoplankton,
bacteria, viruses, and sufficiently small invertebrates
that can traverse the narrow brine passages along the
subgrain boundaries.
Perovich et al.
[1991] perform direct in situ measure-
ments of biomass‐specific diffuse attenuation spectra for
Arctic ice algae. They noted that the in situ attenuation
values to be about three times larger than those obtained
from corresponding in vivo absorption measurements.
